Mixing valve theory
We had our bathroom renovated this year, and this one was a gut job. I didn't do the work this time. About three days ago, my daughter told me that she had lost hot water in the shower. It wasn't dead cold, but there was very little hot. I ran the water, and she was right. It started out warm and got cooler almost immediately. The sink checked out fine, and when I opened up the tub access panel the hot pipe was hot to the touch, so I assumed it was the mixing valve. After a little web research I was able to adjust the valve and now we have hot water. However, I'm now wondering why something like this would happen to a faucet that's only 18 months old, why it would happen so suddenly, and if I should expect the valve or some other component to fail at some point in the near future. I know this is a cliche, but in this 55 year old house it seems like every time I "update" something original (that worked fine for 50 years) with something new, the new stuff just doesn't hold up the same way. Appliances, fixtures, boiler, you name it.
FYI, the faucet/shower was a Delta model purchased at Home Depot for about $150.
If we knew HOW you adjusted the valve, we might have an answer. There are several possibilities, some having NOTHING to do with the valve itself, that can happen spontaneously. The way you start the "story", implies that you might think that if you had done the work yourself it would NOT have happened, but that is not the case.
The symptom you describe could be indicative of a stuck balancing spool, which may have become unstuck when you "adjusted something". But there are lots of other possibilities, so we would have to be there or see some photos of what you have and what you did.